A vertical heat-treating apparatus is used to perform a heat-treating, e.g., thermal oxidation or thermal diffusion, for a plurality of semiconductor wafers (to be referred to as "wafers" hereinafter) all together, or to perform a so-called batch processing. In the heat-treating apparatus of this type, a large number of wafers are loaded in the processing chamber along with a heat-treating boat that holds the wafers in a stacked state with gaps therebetween, and predetermined heat-treating is performed.
FIGS. 35 and 36 are perspective views showing a conventional typical heat-treating boat in the vertical heat-treating apparatus.
FIG. 35 shows a heat-treating boat called a ladder boat. This boat has circular top and bottom plates 16 and 17 that are arranged above a heat-insulating cylinder 4 to vertically oppose each other, and six columns 18 made of, e.g., quartz, to connect the top plate 16 and bottom plate 17 to each other. Grooves 18a are formed in each column 18 to correspond to a plurality of levels where wafers W are to be held horizontally. The wafers W are held as their peripheral edges are inserted in the grooves 18a.
FIG. 36 shows a heat-treating boat called a ring boat. This boat similarly has circular top and bottom plates 16 and 17 that are arranged above a heat-insulating cylinder 4 to vertically oppose each other, and six columns 18 made of, e.g., quartz, to connect the top plate 16 and bottom plate 17 to each other. Ring trays 19 are fixed to the columns 18 to correspond to a plurality of levels where wafers W are to be held horizontally. The wafers W are held as their peripheral portions are placed on the trays 19.
The peripheral edge portions of the wafers are brought into contact with the ring trays 19 to increase the heat capacity of the ring boat, so that the temperature rise/fall speed at the peripheral edge portions can be decreased. Accordingly, even when the temperature rise/fall rate of heat-treating is high, the rate of temperature rise/fall at the central portion and that at the peripheral edge portion of the wafer can be set equal to each other.
As described above, in the ladder boat, each wafer is supported only at the plurality of peripheral edge portions. For this reason, a large moment corresponding to the self weight of the wafer acts on these supported portions to apply an excessive stress to the wafer. When the wafer is heated, the temperature of the peripheral edge portion of the wafer close to the heater tends to increase easily, and a temperature difference tends to occur between the central portion and peripheral edge portion of the wafer.
From these reasons, when the wafer is heat-treated at a high temperature equal to or higher than 1,050.degree. C., a crystal defect called slip tends to occur in the wafer near the supported portions. As the diameter of the wafer increases in recent years, the weight of the wafer itself also increases, and a further countermeasure is required for the slip.
In either the ladder boat or the ring boat, a large load caused by the large number of wafers acts on the columns 18. The softening point of quartz is about 1,400.degree. C. When the wafers are heat-treated at a temperature of, e.g., about 1,000.degree. C., the quartz columns tend to deform by heat. Furthermore, in the ring boat, distortion occurs in the trays 19, and it is difficult to uniformly heat-treat the wafers.
The ring trays 19 of the quartz ring boat are fixed to the columns 18 by welding, i.e., by fusion and adhesion caused by heat. Due to the heat during welding, some distortion may occur in the trays 19 to degrade the flatness, and heat-treating having a high planar uniformity cannot sometimes be performed for the wafers. Hence, welding of the trays 19 requires high precision to make the operation very difficult. As described above, the quartz ring trays 19 are distorted when the temperature becomes about 1,050.degree. C. to interfere with uniform heat-treating of the wafers W. Therefore, the quartz ring trays 19 cannot be used for high-temperature heat-treating.
From the above reasons, a ladder boat and a ring boat made of silicon carbide (SiC) having a higher melting point than that of quartz are also manufactured. However, a heat-treating boat made of SiC is very expensive. Since SiC has a high melting point, a member cannot be adhered to the heat-treating boat by welding, making the manufacture difficult. In other words, in a ring boat made of SiC, ring trays cannot be fixed to the columns by welding.